During hemodialysis (“HD”), the patient's blood is removed from the patient and then passed through a dialyzer of a dialysis machine while also passing a dialysis solution or dialysate through the dialyzer. A semi-permeable membrane in the dialyzer separates the blood from the dialysate within the dialyzer and allows diffusion and osmosis exchanges to take place between the dialysate and the blood stream. These exchanges across the membrane result in the removal of waste products, including solutes like urea and creatinine, from the blood. These exchanges also regulate the levels of other substances, such as sodium and water, in the blood. The cleansed blood is then returned to the patient. In this way, the dialysis machine acts as an artificial kidney for cleansing the blood.
Various other procedures similarly involve removing blood from the patient for treatment or processing and then returning the blood to the patient.